An axial-flow molecular pump having a conventional magnetic bearing is shown in FIG. 1. The axial-flow molecular pump utilizes a magnetic bearing and actively controls each of the five degrees of freedom of the rotating pump member. Reference numeral 1 denotes a rotating axis direction displacement sensor, 2 is a rotating axis direction control electromagnet, 3 is an upper radial direction displacement sensor (two directions of xu, yu), 4 is an upper radial direction control electromagnet (two directions of xu, yu), 5 is a lower radial direction displacement sensor (two directions of xd, yd) and 6 is a lower radial direction control electromagnet (two directions of xd, yd).
Such an axial-flow molecular pump has several drawbacks. A major drawback is that many sensors and electromagnets arranged in a complicated manner are required for the control system in order to effect control with five degrees of freedom of the axial-flow molecular pump for which only high speed rotating operation is required. Accordingly, the calorific value increases owing to a large number of magnets and so on. Further, though the reliability is increased because the pump has no mechanical contact part, the reliability is restricted in view of the complexity of the electric circuit.